When manatees experience prolonged exposure to water temperatures below 68°F (20°C), they can develop a condition called cold-stress syndrome, which can be fatal. The effects of cold stress may be acute, when manatees succumb rapidly to hypothermia, or longer-lasting as chronic debilitation. Chronic cold-stress syndrome is a complex disease process that involves metabolic, nutritional, and immunologic factors. Symptoms may include emaciation, skin lesions or abscesses, fat depletion, dehydration, constipation and other gastrointestinal disorders, internal abscesses, and secondary infections. Over the past 10 years, the Florida Fish and Wildlife Conservation Commission's (FWC) Fish and Wildlife Research Institute (FWRI) has documented several winter seasons (December-April) with higher-than-usual numbers of deaths from cold stress: 2000-2001 (32), 2002-2003 (40), 2003-2004 (48), 2004-2005 (45), 2005-2006 (35) and 2008-2009 (69) (FWRI unpublished data; counts include newborns that might have died from cold stress).

Summary of Winter 2009-2010:

January 2-13, 2010, was the coldest 12-day period on record in parts of south Florida (Naples, West Palm Beach), with temperatures at or below 45°F (7.2°C). In central-east Florida, average daily temperatures during this period were about 15° to 20°F (8° to 11°C) below normal. The latter half of January was warmer, but unusually cold temperatures returned around February 7 and lasted through March. FWC staff made the first cold-related manatee rescue of the year in Duval County on January 4. The first cold-stress-related manatee deaths were reported in Brevard and Broward counties on the east coast on January 7. On January 20, 17 manatees were reported dead from cold stress, a record number reported on a single day. During January-April, 58 manatees were rescued and 503 manatee carcasses were verified in state waters. This carcass count surpassed the record high annual carcass count of 429 in 2009. Mortality was particularly high in the central-east and southwest regions, which include Everglades National Park.

Response actions:

The United States Fish and Wildlife Service and the FWC consulted the Working Group on Marine Mammal Unusual Mortality Events, a component of the National Oceanic and Atmospheric Administration's Marine Mammal Health and Stranding Response Program. On February 26, the Working Group declared the event to be an Unusual Mortality Event (UME). Prior to the official declaration, the response was under way, as the FWC had assembled a response team to direct the course of the investigation, maintain close communication among response team leaders, provide information to partner agencies and organizations, and contribute to daily operations. Sub-teams included rescue/carcass-salvage logistics, environmental sampling, aerial survey and ground monitoring, data management/GIS, necropsy/medical, and media relations. Throughout the cold event, law enforcement officers, biologists, and volunteers recovered manatee carcasses. FWC staff and volunteers transported most of them to FWRI's Marine Mammal Pathobiology Laboratory for necropsy (non-human autopsy).

Cold-stress mortality:

As of December 5, preliminary necropsy data indicate 244 cold-stress-related deaths in 2010, which is almost ten times the five-year average (27) for the cold stress category for the same portion of the year (January 1 - December 5). Thirteen of these cold-related deaths were newborns. A record number of 172 cold-related deaths were reported between January 11 and February 9. Most of these manatees appeared to have died from acute hypothermia, not from chronic debilitating effects of the cold. This acute cold shock was seen in manatees of all sizes; chronic cold-stress syndrome usually affects only juveniles. From mid- February through the first week of April, the carcass count remained above average, which was mostly attributed to chronic conditions from exposure to cold. Undetermined causes of death and unrecovered (but verified) carcass numbers for January-March 2010 were also at a record high (120 and 64, respectively). The similarity of the timing and location of these carcasses and the cold-stress carcasses suggests that the majority of the deaths from unknown causes were also due to cold stress.

Population impact:

The high number of manatee deaths and their wide geographic extent are of concern, but this and other cold-weather events are natural occurrences. Our understanding of manatee population dynamics is based on a combination of ecological information and mathematical models. To evaluate the effect of this UME on the Florida manatee population, scientists will need time to observe and analyze the survival rates of the adult manatees they monitor. The FWC and partner organizations will use this information to better understand the roles of various threats and to improve our ability to forecast population changes. This UME underscores the importance of warm water to the species. Loss of warm-water habitat is a significant long-term threat to the manatee population. Adequate warm-water habitat must be effectively managed in order to protect Florida's manatees.

Distribution of manatee carcasses reported in Florida, January 11-April 9, 2010. These dates mark the extent of the 2010 cold-related manatee mortality event. Manatees in all regions except the northwest suffered unusually high mortality.

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